The objectives of the bioanalysis core are to measure the opioid receptor selectivity of the peptide analogues under development and evaluate their ability to reach CNS opioid receptors after peripheral administration. The procedures described are designed to allow the screening of a large number of analogues in a short period of time to prevent delays in the development cycle. The specific aims are: 1. To measure the binding affinity of each of the analogues at mu, delta and kappa opioid receptors. Binding isotherms will be evaluated for multiple site interactions suggestive of the presence of opioid receptor subtypes. 2. To determine the in vitro potency of the analogues in the well characterized mouse vas deferens and guinea pig ileum bioassays. 3. To measure the ability of the analogues to cross membrane barriers both in vivo after parenteral administration and in vitro using a model of the mammalian blood brain barrier. The endpoint measured in the in vivo studies will be the induction of analgesia to a thermal stimulus while that measured in vitro will be the amount of analogues under study penetrating the model membrane barrier.